1. Field of the Invention
This invention relates to electrical terminals and, more specifically, to electrical terminals for high voltage insulated cables such as those used with aircraft ignition systems.
2. Prior Art
Terminals for cables used in aircraft ignition systems are subject to extreme operating conditions. The very high voltages used can cause burning and deterioration of the electrical contacts in the terminals if adequate electrical contact is not made between the mating parts. In addition, the reduced atmospheric pressure in which these systems operate increases the tendency for flashover or short-circuiting to occur. Furthermore, the high temperatures to which the terminals are exposed accelerates the deterioration of the seals used to prevent flashover.
Standards have been set for terminals and the receptacles with which they mate to ensure the compatability of parts made by different manufacturers. One type of receptacle used with high voltage aircraft ignition systems is the ARP 670-4F. This receptacle has an electrical contact recessed in an insulator sleeve which in turn is recessed in an externally threaded metallic sleeve. This receptacle is designed to mate with an ARP 670-4M terminal. This standard terminal has an electrical contact which extends beyond the end of an elongated insulator dimensioned to be received in the recess in the insulator sleeve of the receptacle. An internally threaded coupling nut which is retained by a shoulder on a ferrule forming part of the terminal threadedly engages the metallic sleeve on the receptacle to securely mate the terminal and the receptacle.
While the standards set the dimensions and the tolerances for the mating parts of the receptacle and terminal, the internal arrangement for the seals and means for biasing the terminal contact toward the receptacle contact are not specified. In the present terminal made by the assignee of the present invention, a resilient grommet slidable on the elongated insulator of the terminal is compressed against the end of the insulator sleeve on the receptacle by a helical compression spring encircling the terminal insulator and the cable. A second helical compression spring bears against a resilient bushing abutting the end of the terminal insulator to urge the terminal contact secured to the end of the terminal insulator against the receptacle contact. By providing two springs, the contacts and the sealing grommets may both be seated with the required force despite variations in the tolerances of the parts of both the terminal and the receptacle. Captivation assemblies are utilized to pre-load the springs which in turn keep the overall terminal assembly shorter.
While the above mentioned terminal has been successfully used over a period of about fifteen years, there is room for improvement. Interference can occur between the grommet and the inner spring which inhibits movement of the terminal insulator, allowing an air gap to be present between the supposedly mated contacts which results in contact burning. This interference also increases the insertion force required to mate the terminal with a receptacle. Furthermore, interference between the grommet and the mainspring precludes the grommet from sliding freely over the terminal insulator so that the correct pressure is not applied through the grommet, thereby increasing the probability of flashover at altitude. Another problem area with the present design is deterioration of the sealing grommet and bushing when they have been subjected to high temperatures. The bushing tends to vulcanize to the jacket of the cable and the grommet vulcanizes to either the ceramic sleeve or to the mating receptacle. When unmating, the vulcanized grommet tears apart or remains in the receptacle necessitating more extensive field repair.
It is an object of the present invention to provide a simple, reliable high voltage terminal.
It is another object of the invention to provide a high voltage terminal which has a longer life and is less costly to manufacture than present terminals.
It is yet another object of the present invention to reduce the incidence of high insertion forces, high altitude seal failure and contact burning in high voltage terminals.
More specifically, it is an object of the present invention to eliminate interference between the sealing grommet and the spring assembly in high voltage terminals currently used.
It is also a specific object of the present invention to eliminate one spring and the spring captivation assemblies of the high voltage terminals currently in use.
It is another specific object of the invention to slow down the deterioration of the grommets and bushings of high voltage terminals under environmental stress.